I/C package / thermal-solution retention mechanism with spring effect

ABSTRACT

A thin, lightweight retention mechanism with a spring force holds an integrated circuit package to a circuit board. The retention mechanism consists of a pressure plate, a backing plate, and a fastening means for applying a deforming force to the plates, such as screws and nuts. The plates are paraboloid or dish-shaped and made of an elastically deformable material, such as steel. The fastening means simultaneously applies deforming forces to the peripheries of the plates to create a continuous spring force to effect electrical continuity between the integrated circuit package and the circuit board. In addition, a method of testing the retention mechanism and a method of assembling the retention mechanism are disclosed.

TECHNICAL FIELD OF THE INVENTION

Generally, the present invention relates to electronics packaging and,in particular, the present invention relates to a retention mechanismfor mounting an integrated circuit package to a circuit board.

BACKGROUND

Today there is an increased demand for lightweight and thin electronicdevices. This demand has encouraged inventors to look for ways toeliminate or reduce heavy and thick components in electronics packaging.One part of electronics packaging is mounting an integrated circuit to acircuit board. Traditionally, an integrated circuit package was mountedto a circuit board by inserting pins on the integrated circuit packageinto holes on a connector and then attaching the connector to a circuitboard. As the number of pins on integrated circuit packages increasesand available space decreases, it is more difficult to insert leads orwires through holes and it requires loose wires and solder.

Traditionally, flat, heavy, and thick plates were used to apply highloads to connectors. The high loads are sometimes necessary to get lowcontact resistance, which provides good conductivity from the integratedcircuit to the circuit board. As a result of the high loads, someconnectors and circuit boards warped and were rendered useless.Traditionally, very heavy and thick backing plates were used to try toprevent warping. This is not acceptable for the lightweight and thindevices consumers demand today.

For the reasons stated above, and for other reasons stated below whichwill become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a significant need inthe art for a thin, lightweight retention mechanism with a spring forceto hold an integrated circuit package to a circuit board.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a top view of one embodiment of the present invention.

FIG. 2 shows a top view of one embodiment of a pressure plate.

FIG. 3 shows a bottom view of one embodiment of the present invention.

FIG. 4 shows a top view of one embodiment of a backing plate.

FIG. 5 shows an exploded view of one embodiment of the presentinvention.

FIG. 6 shows a sectional view of one embodiment of the presentinvention.

FIG. 7 shows a sectional view of another embodiment of the presentinvention.

FIG. 8 shows an exploded view of one embodiment of the presentinvention.

FIG. 9 shows a flowchart of one method of testing the present invention.

FIG. 10 shows a flowchart of one method of assembling the presentinvention.

DETAILED DESCRIPTION

In the following detailed description of the invention reference is madeto the accompanying drawings which form a part hereof, and in which isshown, by way of illustration, specific embodiments in which theinvention may be practiced. In the drawings, like numerals describesubstantially similar components throughout the several views. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention. Other embodiments may be utilizedand structural, logical, and electrical changes may be made withoutdeparting from the scope of the present invention.

FIG. 1 shows a top view of one embodiment of the present invention. (Fora bottom view, see FIG. 3.) One embodiment of the present invention is aretention mechanism 100 for mounting an integrated circuit package 102to a circuit board 104. The retention mechanism 100 comprises a pressureplate 200, a backing plate 400, and a means for applying forces to theplates 106. (The backing plate 400 is shown in the bottom view of FIG.3.) The integrated circuit package 102 is held and retained to thecircuit board 104 by a continuous spring force. The shape of the platesprovides a continuous spring force regardless of the variation of thestack-up with time. The retention mechanism 100 is extremely robust andresponsive to expansion and contractions of the materials produced bytemperature excursions and mechanical loads.

The integrated circuit package 102 may contain a microprocessor, anapplication-specific integrated circuit (ASIC), a cache memory, a mainmemory, other integrated circuit (IC), or multiple electronic products.The integrated circuit package 102 may be any type of package, such as aplastic quad flat pack (PQFP), a Swiss outline package (SOP), a Swissoutline J-leaded (SOJ), a small outline IC (SOIC), a plastic leaded chipcarrier (PLCC), a thin quad flat pack (TQFP), a thin small outlinepackage (TSOP), a direct chip attach on a printed circuit board(DCAPCB), a chip scale package (CSP), a tape carrier package (TCP), aplastic pin grid array (PPGA), a ceramic pin grid array (CPGA), aplastic ball grid array (PBGA), a ceramic ball grid array (CBGA), a tapeball grid array (TBGA), a metal ball grid array (MBGA), or an area-arraysolder-bumped flip-chip technology.

FIG. 2 shows a top view of one embodiment of a pressure plate 200. Inone embodiment, the pressure plate 200 is a dish-shaped elasticallydeformable plate that has an apex (also referred to as a summit) 202 anda periphery 204. The pressure plate 200 may be deformed by applying aforce 206 at the periphery 204, resulting in a force generally directedtowards the apex 202. The force 206 is preferentially directed towardsthe apex 202 to avoid warping the circuit board 104.

FIG. 3 shows a bottom view of one embodiment of the present invention.In FIG. 3, a backing plate 400 is shown with an irregularly shapedperiphery 404. In one embodiment, the periphery 404 of the backing plate400 or the pressure plate 200 may be fractal-shaped. In one embodiment,the backing plate 400 is in contact with a circuit board 104 whereadditional components are attached to its back side after the integratedcircuit package 102 has been mounted to the circuit board 104. In thiscase, the fractal-shaped periphery 404 enables or facilitates theirattachment.

FIG. 4 shows a top view of one embodiment of a backing plate 400, whichis similar to the pressure plate 200 in FIG. 4. Like the pressure plate200, the backing plate 400 is a dish-shaped, elastically deformableplate that has an apex 402 and a periphery 404. The backing plate 400may also be deformed by applying a force 406 at the periphery 404,resulting in a force generally directed towards the apex 402. Once theretention mechanism 100 is assembled, the backing plate 400 acts as aloaded spring providing a constant load over the life of the product.Both the pressure plate 200 and the backing plate 400 are made from amaterial selected from the group consisting of beryllium copper andsteel. These materials have a modulus to provide a large spring force,even with a lightweight and thin mechanism.

The pressure plate 200 and the backing plate 400 may be similarlyshaped, but are oriented differently in the present invention, as shownin FIG. 5. FIG. 5 shows an exploded view of one embodiment of thepresent invention. In FIG. 5, the pressure plate 200 curves upward,while the backing plate 400 curves downward. Both have their convexsurfaces inward. (See FIG. 2, element 210 and FIG. 4, element 408). Theshape of the plates helps create a spring force in the retentionmechanism 100. The force 206 on the pressure plate 200 and the force 406on the backing plate 400 are applied simultaneously to the periphery 204of the pressure plate 200 and the periphery 404 of the backing plate.These deforming forces engage the apex 202 of the pressure plate 200with a surface of the integrated circuit package 102 and engage the apex402 of the backing plate 400 with a surface of the circuit board 104. Ameans 106 for applying the deforming forces is shown in FIG. 5 asthreaded studs with nuts, but could be any fastener, such as screws,c-clamps, and the like. Thus, the pressure plate 200 and the backingplate 400 keep a sandwich of components together. For example, as thescrews are tightened, the deformation of the plates will starttransferring a load to the sandwich. As a result, there is continuouselectrical conductivity between the integrated circuit package 102 andthe circuit board 104. In one embodiment, an average contact resistancebetween the integrated circuit package 102 and the circuit board 104 isless than about 50 milliohms, preferably about 13 milliohms.

This sandwich of components, shown in FIG. 5 may additionally include agasket 500 and a connector 504, in some embodiments of the presentinvention. The gasket 500 is positioned between the pressure plate 200and the integrated circuit package 102. The gasket 500 is made of anelastically deformable material, such as an elastomer sheet. The gasket500 helps to distribute the spring force evenly across the contacts ofthe integrated circuit package 102 and the circuit board 104. The gasketalso helps prevent damage to the integrated circuit package 102. In oneembodiment, the gasket has a height of less than about 2 millimeters,preferably less than about 1 millimeter. The connector 504 is interposedbetween the integrated circuit package 102 and the circuit board 104.The connector 504 may be, a ball grid array, a land grid array, chipscale package, or any other type of connecting device or gasket. In oneembodiment, the connector 504 has a height less than about 2millimeters, preferably less than about 1 millimeter. In one embodiment,the integrated circuit package 102 is pinless and the connector 504 iswithout pin holes.

As shown in FIG. 5, one or more of the pressure plate 200, the backingplate 400, or the gasket 500 may define one or more windows. A window isshown in the center of these parts in FIG. 5, but could be locatedelsewhere, and there could be multiple windows in any part. In FIG. 5,the windows expose the integrated circuit, but could expose any selectedportion of the plates or circuit board 104. These windows can aid intesting or fitting additional parts in an electronic device. FIG. 8, anexploded view of one embodiment of the present invention, shows theseparts without windows.

FIG. 6 shows a sectional view of one embodiment of the presentinvention. In one embodiment, a retention mechanism 100 comprises apressure plate 200, an integrated circuit package 102, a circuit board104, a backing plate 400, and one or more fasteners 106. The retentionmechanism 100 may further comprise a connector 504 and a gasket 500. Theforce 206 applied to the pressure plate 200 and the force 406 applied tothe backing plate result in the continuous spring force retaining thesandwich together, is also shown in FIG. 5.

Referring back to FIG. 2, the pressure plate 200, in one embodiment, isa paraboloid, elastically deformable plate with a concave surface 208, aconvex surface 210, a summit 202, and a periphery 204 spaced away fromthe summit 202. A paraboloid plate may be dish-shaped or slightly bowed.A dish-shaped plate is a paraboloid of revolution, i.e. in threedimensions. A slightly bowed plate is a paraboloid in two dimensions.While the shape may be a parabola or similar to one, it need not be andcould be any simple curve, even circular, elliptic, hyperbolic, or thelike, and may, as well, comprise any portion of any of these curves. Thepressure plate 200 may be deformed by applying a force 206 to theperiphery 204 directed generally towards the summit 202. The shape ofthe plate helps direct the force towards the summit 202, but the forceitself may be applied generally downward.

FIG. 7 shows a sectional view of another embodiment of the presentinvention. A difference between FIG. 6 and FIG. 7 is the heat sink 700.In one embodiment, a heat sink 700 is in contact with the concavesurface 208 of the pressure plate 200. In one embodiment, the retentionmechanism 100 comprises a heat sink, a backing plate, and a means 106for applying a deforming force without a pressure plate 200. If theintegrated circuit package 102 includes an organic substrate, then boththe pressure plate 200 and the backing plate need to be used, but if theintegrated circuit package 102 is a rigid ceramic package, then the heatsink can be attached directly to the package. Another difference is theconnector 504 has pins in FIG. 7 while it is pinless in FIG. 6.

In one embodiment, the height of the pressure plate 200 is less thanabout 2 millimeters, preferably less than about 1.5 millimeters. Theheight is measured from the summit of the pressure plate 200 to theperiphery 204 of the pressure plate 200. The summit of the pressureplate 200 is located on the convex surface 210 of the pressure plate200. The periphery 204 of the pressure plate 200 is located on theconcave surface 208 of the pressure plate 200. (See FIG. 2). Similarly,in one embodiment, the height of the backing plate is also less thanabout 2 millimeters, preferably less than about 1.5 millimeters. Theretention mechanism 100 is able to provide high forces, such as 70pounds that are needed to achieve the low thermal resistance thattoday's devices need, but without the thick, heavy plates traditionallyused. The thin pressure plate 200 and thin backing plate 400 provide acontinuous spring force evenly distributed across the contacts of theelectrical parts, even if a thermal interface material compresses. Thebond-line thickness of thermal interface material is very susceptible toload variations. The retention mechanism 100 holds the pressure on thethermal interface material over the life of the product.

The integrated circuit package 102, in one embodiment, is sandwichedbetween the pressure plate 200 and the circuit board 104, so that itstop surface is in contact with the convex surface 210 of the pressureplate 200 and its bottom surface is in contact with the circuit board104. In another embodiment, the integrated circuit package 102 issandwiched between the gasket 500 and the connector 504 so that its topsurface is in contact with the gasket 500 and its bottom surface is incontact with the connector 504. This embodiment is shown in FIG. 6,including a gasket 500 and a connector 504. In one embodiment, theintegrated circuit package 102 includes an organic land grid array. Inanother embodiment, the integrated circuit package 102 includes a flipchip pin grid array.

The circuit board 104, in one embodiment, is sandwiched between theintegrated circuit package 102 and the backing plate 400, so that itstop surface is in contact with the integrated circuit package 102 andits bottom surface is in contact with the backing plate 400. In anotherembodiment, the circuit board 104 is sandwiched in between the connector504 and the backing plate 400, so that its top surface is in contactwith the connector 504 and its bottom surface is in contact with thebacking plate 400. This embodiment is shown in FIG. 6, including aconnector 504.

The backing plate 400, in one embodiment, is a paraboloid, elasticallydeformable plate with a concave surface 408, a convex surface 410, asummit 402, and a periphery 404 spaced away from the summit 402. Thepressure plate 200 may be deformed by applying a force 406 to theperiphery 404 directed generally towards the summit. The force 406 onthe backing plate 400 opposes the force 206 on the pressure plate 200.

One or more fasteners 106, in one embodiment, simultaneously apply theforce 206 on the pressure plate 200 and the force 406 on the backingplate 400, deforming the plates. The forces engage the top surface ofthe integrated circuit package 102 with the convex surface 210 of thepressure plate 200 and the bottom surface of the circuit board 104 withthe convex surface 410 of the backing plate 400. These forces deform theplates so as to effect continuous electrical continuity between theintegrated circuit package 102 and the circuit board 104.

Another aspect of the present invention is an electronic assembly. Anelectronic system is broadly defined as any product comprising anelectronic assembly. Examples of electronic systems include computers(e.g., desktop, laptop, hand-held, server, etc.), wirelesscommunications devices (e.g., cellular phones, cordless phones, pagers,etc.), computer-related peripherals (e.g., printers, scanners, monitors,etc.), entertainment devices (e.g., televisions, radios, stereos, tapeand compact disc players, video cassette recorders, Motion PictureExperts Group, Audio Layer 3 (MP3) players, etc.), and the like.

In one embodiment, an electronic assembly comprises a pressure plate200, an integrated circuit package 102, a circuit board 104, a backingplate 400, and a means 106 for applying force to deform the plates. Inone embodiment, the electronic assembly comprises a retention mechanism100 retaining a sandwich having layers from top to bottom: a pressureplate 200, an integrated circuit package 102, a circuit board 104, and abacking plate 400. Each layer is in contact with the next, effectingcontinuous electrical continuity between the integrated circuit package102 and the circuit board 104.

In another embodiment, the electronic assembly comprises a retentionmechanism 100 retaining a sandwich having layers from top to bottom: apressure plate 200, an integrated circuit package 102, a connector 504,a circuit board 104, and a backing plate 400. Each layer is in contactwith the next, effecting continuous electrical continuity between theintegrated circuit package 102 and the circuit board 104.

In another embodiment, the electronic assembly comprises a retentionmechanism 100 retaining a sandwich having layers from top to bottom: apressure plate 200, a gasket 500, an integrated circuit package 102, aconnector 504, a circuit board 104, and a backing plate 400. Each layeris in contact with the next, effecting continuous electrical continuitybetween the integrated circuit package 102 and the circuit board 104.

Another aspect of the present invention is a method of testing aretention mechanism. FIG. 9 shows a flowchart of one method of testingthe present invention 900. The retention mechanism 100 is tested toassess the quality of the contact between the integrated circuit package102 and the circuit board 104 or between the integrated circuit package102 and the circuit board 104. During the test, resistance is measuredusing a chain called a daisy chain. To set up the test, in oneembodiment, a cable goes through the circuit board 104, into theintegrated circuit package 102, into the die, and back again to adifferent part of the circuit board 104. To set up the test, in anotherembodiment, a cable goes through the circuit board 104, through theconnecter 504, into the integrated circuit package 102, into the die,and back again through a different contact in the connector 504 and adifferent part of the circuit board 104. In one embodiment, a method oftesting a retention mechanism 900 comprises: creating a daisy chain froma circuit board through a connector into a integrated circuit package toa die and back to the circuit board 902 and measuring resistance betweenthe die and the circuit board 904. In another embodiment, the testincludes measuring the resistance between the integrated circuit packageand the circuit board 906. In one embodiment, the method of testing 900further comprises determining the contribution of the connector to thetotal resistance 908 and determining the extent of contact between theconnector and the integrated circuit package 910. In testing theretention mechanism 100, contact was achieved in all the daisy chainsand a very low average contact resistance of about 13 milliohms wasmeasured.

Another aspect of the present invention is a method of assembling aretention mechanism. FIG. 10 shows a flowchart of one method ofassembling the present invention 1000. In one embodiment, the method1000 comprises: placing an integrated circuit package on a circuit board1002, placing a slightly curved pressure plate on a top surface of theintegrated circuit package 1004, placing a slightly curved backing plateon a bottom surface of the circuit board 1008, and applying force toouter edges of the plates to retain the integrated circuit package onthe circuit board and to create an evenly distributed pressure acrossconductors of the integrated circuit package 1010. In anotherembodiment, the method 1000 further comprises placing a heat sink on theslightly curved pressure plate 1012 and attaching the heat sink to thecircuit board 1014. In another embodiment, the method 1000 furthercomprises placing a gasket between the integrated circuit package andthe slightly curved pressure plate 1006.

It is to be understood that the above description is intended to beillustrative, and not restrictive. Many other embodiments will beapparent to those of skill in the art upon reviewing the abovedescription. The scope of the invention should, therefore, be determinedwith reference to the appended claims, along with the full scope ofequivalents to which such claims are entitled.

What is claimed is:
 1. A retention mechanism for mounting an integratedcircuit package to a circuit board, comprising: a dish-shaped,elastically deformable pressure plate, having a first apex and a firstperiphery spaced away from the first apex, the pressure plate beingdeformable by applying a first force to the first periphery directedgenerally towards the first apex: a dish-shaped, elastically deformablebacking plate, haying a second apex and a second periphery spaced awayfrom the second apex, the backing plate being deformable by applying asecond force to the second periphery directed generally towards thesecond apex: and means for simultaneously applying the first and seconddeforming forces to the first and second peripheries to engage the firstapex with a surface of the integrated circuit package and the secondapex with a surface of the circuit board so as to effect continuouselectrical continuity between the integrated circuit package and thecircuit board, wherein an average contact resistance between theintegrated circuit package and the circuit board is less than about 50milliohms.
 2. The retention mechanism of claim 1 wherein the averagecontact resistance is about 13 milliohms.
 3. The retention mechanism ofclaim 1 further comprising an elastically deformable gasket positionedbetween the pressure plate and the integrated circuit package.
 4. Theretention mechanism of claim 1 wherein the plates are made from amaterial selected from the group consisting of beryllium copper andsteel.
 5. The retention mechanism of claim 3 wherein the gasket has aheight less than about 2 millimeters.
 6. The retention mechanism ofclaim 5 wherein the gasket has a height less than about 1 millimeter. 7.A retention mechanism comprising: a paraboloid, elastically deformablepressure plate, having a concave surface, a convex surface, a summit,and a periphery spaced away from the summit, the pressure plate beingdeformable by applying a first force to the periphery directed generallytowards the summit: an integrated circuit package having a top and abottom surface, the convex surface of the pressure plate beingcontactable by the top surface: a circuit board having a ton and abottom surface, the bottom surface of the integrated circuit packagebeing contactable by the top surface: and a paraboloid, elasticallydeformable backing plate, having a concave surface, a convex surface, asummit, and a periphery spaced away from the summit, the backing platebeing deformable by applying a second force, opposing the first force,to the periphery of the backing plate directed generally towards thesummit of the backing plate, the bottom surface of the circuit boardbeing contactable by the convex surface of the backing plate; and one ormore fasteners to simultaneously apply the first and second deformingforces to the peripheries of the plates to engage the ton surface of theintegrated circuit package with the convex surface of the pressure plateand the bottom surface of the circuit board with the convex surface ofthe backing plate and to deform the plates so as to effect continuouselectrical continuity between the integrated circuit package and thecircuit board, wherein the summit of the pressure plate is located onthe convex surface of the pressure plate and the periphery of thepressure plate is located on the concave surface of the pressure plate,and the height from the summit of the pressure plate to the periphery ofthe pressure plate is less than about 2 millimeters.
 8. The retentionmechanism of claim 7 wherein the height from the summit to the peripheryis less than about 1.5 millimeters.
 9. The retention mechanism of claim7 wherein the summit of the backing plate is located on the concavesurface of the backing plate and the periphery of the backing plate islocated on the convex surface of the backing plate, and the height fromthe periphery to the summit is less than about 2 millimeters.
 10. Theretention mechanism of claim 7 wherein the integrated circuit packageincludes an organic land grid array.
 11. The retention mechanism ofclaim 7 wherein the integrated circuit package includes a flip chip pingrid array.
 12. The retention mechanism of claim 9 wherein the heightfrom the periphery to the summit is less than about 1.5 millimeters. 13.An electronic assembly comprising: a paraboloid, elastically deformablepressure plate, having a concave surface, a convex surface, a summit,and a periphery spaced away from the summit, the pressure plate deformedby first force applied to the periphery directed generally towards thesummit; an integrated circuit package having a top and a bottom surface,the top surface in contact with the convex surface of the pressureplate; a circuit board having a top and a bottom surface, the topsurface in contact with the bottom surface of the integrated circuitpackage; and a paraboloid, elastically deformable backing plate, havinga concave surface, a convex surface, a summit, and a periphery spacedaway from the summit, the backing plate deformed a second force,opposing the first force, applied to the periphery of the backing platedirected generally towards the summit of the backing plate, the convexsurface of the backing plate in contact with the bottom surface of thecircuit board; means for simultaneously applying the first and seconddeforming forces to the periphery of the pressure plate and theperiphery of the backing plate to engage the convex surface of thepressure plate with the top surface of the integrated circuit packageand the convex surface of the backing plate with the bottom surface ofthe circuit board and to deform the plates so as to effect continuouselectrical continuity between the integrated circuit package and thecircuit board; and a connector interposed between the integrated circuitpackage and the circuit board, wherein an average contact resistancebetween the integrated circuit package and the circuit board is lessthan about 50 milliohms.
 14. The electronic assembly of claim 13 furthercomprising an elastically deformable gasket interposed between thepressure plate and the integrated circuit package.